Most ROP proteins so far characterized show clear homology to protein kinases 
. Some are active kinases, while others are predicted to be pseudokinases—i.e., catalytically inactive. The best-studied active kinases are ROP16 and ROP18. ROP16 is a tyrosine kinase that mimics the action of host JAKs by phosphorylating the key tyrosine needed for activation of STATs 
. This is an extremely rapid process such that within one minute of the commencement of invasion, STAT3 and STAT6 (at least) are activated and translocated to the nucleus where they turn on many immune response genes and downregulate the expression of others.
ROP18 is a serine/threonine kinase whose targets include the interferon-gamma-inducible p47 GTPases encoded by immune-response genes (IRGs; 
). In the absence of ROP18, IRGs can multimerize on the PVM, somehow causing it and the parasites within to be destroyed. ROP18's phosphorylation of IRGs occurs within the nucleotide-binding site, causing them to lose the ability to oligomerize and, thus, to lose activity. ROP18 also has been reported to bind and inactivate ATF6β, a transcription factor with a role in the interferon-gamma response 
. A complete inventory of the targets of ROP16 and ROP18 has yet to be generated, but it is likely an extensive list.
The complete sequence of the Toxoplasma
genome reveals that, in addition to ROP16 and ROP18, the ROP kinase family comprises more than 30 additional genes, many of which are under diversifying selection 
. One of these, ROP38, is represented by three tandemly duplicated genes and appears to play a role in modulating MAPK signaling in host cells 
. The actual target for the ROP38 kinase has yet to be determined.
Within the overall family of ROP “kinases,” most are predicted to be pseudokinases. Interestingly, these have been subject to gene duplication such that clusters of 3–12 nearly identical genes are often found at a given chromosomal location. The function of most of these pseudokinases is not yet known, but one, ROP5, has recently been shown to play a key role in the strain-specific differences in virulence in mice 
. Two nonmutually exclusive mechanisms for how ROP5 functions have been described. In the first, ROP5 binds to and thereby interferes with the oligomerization of the IRGs mentioned above 
. This both interferes with their action and makes them more susceptible to phosphorylation and, thus, permanent inactivation by ROP18. The second mechanism involves ROP5's ability to directly regulate the catalytic activity of ROP18 by allosteric means and independent of the substrate being phosphorylated 
Many of the ROP kinase family, including ROP5 and ROP18, are found specifically associated with the PVM. This association is mediated by an arginine-rich, amphipathic, helical domain found toward the N-terminus of the mature protein 
. At least in the case of ROP18, the proper localization provided by this domain is required for biological function—i.e., ROP18's ability to phosphorylate IRGs 
. The N-terminal region is also necessary for ROP18's ability to bind to ATF6β